1. Field of the Invention
The present invention pertains to a flow sensor for measuring the flow of fluid through a conduit, and, in particular, to a flow sensor having a flow resistive element and housing supporting the flow resistive element that optimizes the performance of the flow sensor while minimizing its components and cost.
2. Description of the Related Art
There are numerous situations where it is desirable to measure the flow of fluid passing through a conduit. For example, it is known to assess the pulmonary function of a patient by monitoring the flow and volume of fluid inhaled and exhaled by that patient using a spirometer. In its simplest form, a spirometer is a fluid carrying conduit that the patient breathes into, and a flow sensor for measuring the flow of fluid passing through the conduit. Examples of conventional spirometers are taught in U.S. Pat. Nos. 5,107,860 to Malouvier et al., 5,137,026 to Waterson et al., and 5,722,417 to Garbe.
It is also known to treat a breathing disorder, such as sleep apnea, with a pressure support system that delivers a flow of breathing gas to the airway of a patient at an elevated pressure. A typical conventional pressure support system 30 is shown in FIG. 1 and includes a pressure generating system 32 and a patient circuit 34, which includes a conduit 36 and a patient interface 38. Pressure generating system 32 receives a supply of breathing gas, such as air, and elevates the pressure of the breathing gas at its output to a pressure that is greater than atmospheric pressure for delivery to the patient.
Depending on its operating capabilities, pressure generating system 32 may include a pressure control system to control the pressure of fluid delivered to the patient. Examples of suitable pressure control mechanisms include (1) a pressure control valve 40 downstream of the pressure generator, (2) a variable speed motor 42 associated with pressure generator 32, or both to vary the pressure output by the pressure generator. The pressure control valve and variable speed motor typically operate under the control of a control unit 44 in a feedback fashion based on signals from sensors associated with the patient circuit. A pressure support system that provides a variable pressure to the patient based on patient's respiratory cycle, for example, is taught in U.S. Pat. Nos. 5,148,802 and 5,433,193, both to Sanders et al., the contents of which are incorporated by reference into the present application.
Typically, a flow sensor 46 is provided to measure a rate at which the breathing gas flows within conduit 36. It is also known to provide a pressure sensor 48 that detects the pressure of the gas in the patient circuit or at the patient. In the illustrated embodiment, pressure sensor 48 is in fluid communication with patient interface device 38 via conduit 36. A conventional pressure support system typically includes an input/output interface device 50, such as a keypad and/or display, for communicating, information, data and/or instructions between the user and control unit 44.
Numerous sensors exist that function as flow sensor 46, i.e., to measure the flow of fluid in the patient circuit. For example, it is known to use a pneumotach flow meter placed directly in the patient circuit to measure the flow of fluid. Examples of conventional flow meters are taught in U.S. Pat. Nos. 4,083,245 to Osborn; 4,796,651 to Ginn et al.; 4,905,709 to Bieganski et al.; 4,989,456; 5,033,312 and 5,038,621 all to Stupecky; and 5,357,792 to Norlien et al.
It should be noted that the term “fluid” as used herein refers to any gas, including a gas mixture or a gas with particles, such as an aerosol medication, suspended therein. Most commonly, the fluid delivered to a patient by a pressure support system is pressurized air.
As with the spirometers noted above, a conventional flow sensor typically includes a conduit having a flow element disposed in the conduit to provide a known resistance to flow through the sensor, thereby creating a pressure differential across the flow element. In a first type of conventional flow sensor, most of the fluid flowing through the sensor passes through the flow element, and the pressure differential created by the flow element causes a lesser portion of the gas passing through the sensor to be diverted through a bypass channel connected across the flow element. An airflow sensor in the bypass channel measures the flow of gas passing therethrough. Because the area of the flow element and the area of the bypass channel are known and fixed relative to one another, the amount of gas flowing through the bypass channel is a known fraction of the total gas flow delivered to the flow sensor. Thus, the flow of fluid through the flow sensor is determined from the bypass flow measurement.
In a second type of conventional flow sensor, a pressure sensor, rather than an airflow sensor, is provided in the bypass channel. Gas does not pass through the pressure sensor. Instead, each side of a diaphragm in the pressure sensor communicates with respective pressures on either side of the flow element. The pressure sensor measures the pressure differential across the flow element to determine the rate of flow of gas through the flow sensor.
A common goal of conventional flow sensors is to provide a linear relationship between the flow of fluid through the sensor and the pressure difference developed across the flow element as a result of this flow. To this end, numerous conventional flow sensors employ a flow element that has a variable geometry, such as bending flaps, that alter their shape, and, hence, the pressure-flow relation, based on the rate of flow through the flow sensor.
Yet another goal of conventional flow sensors is to provide a laminar flow of fluid through the sensor, because turbulent flow can produce erroneous flow measurements, especially if the turbulence is located at the point in the flow sensor where the pressure sensor or mass airflow sensor communicates with the primary flow of gas through the flow sensor. To this end, it is known to employ flow element in the flow sensor that is comprised of a large number of honey-comb like channels that extend in the direction of gas flow. These numerous channels straighten or make laminar the flow of gas through the flow element, thereby preventing or minimizing turbulence.
A significant disadvantage of flow sensors that use a variable orifice (flexible obstruction) flow element is the complexity of such designs. That is, the bending elements must be precisely manufactured and assembled to produce the correct pressure-flow relationship. In addition, some of these types of flow sensor are position dependent and may produce turbulent flow due to their lack of flow laminarizing elements.
A significant disadvantage of conventional flow sensors that attempt to make laminar the flow of fluid through the flow element is the complexity and cost of manufacturing a flow element having a large number of a honey-comb like channels. Also, these channels are prone to clogging, and often provide a less than ideal pressure-flow relationship.